1. Field of the Invention
This invention relates to the art of forming or shaping end-use objects from thermoformable materials.
2. Description of the Prior Art
Structural panels are used for many applications. For certain applications it is desired that the panel have both rigidity and structural strength and that it also be relatively light-weight. In order to provide such panels various techniques have been devised for providing a core of rigid material having a relatively low density between skins of material which have relatively high densities. Most of these techniques require the use of specific adhesives for the purpose of bonding the low density cores to the high density skin laminae. This adhesive bonding technique has many drawbacks. One such drawback lies in the fact that it is not always possible to find an adhesive which is capable of readily bonding the two materials which one would desire to place in a laminate of this type. In order to provide such adhesives for such different laminae, it is necessary in some cases to devise relatively expensive adhesives. Considerable time and effort, moreover, is involved in utilizing some adhesives in the fabrication of the laminates. Many of these adhesives, for example, contain solvents. During the manufacture of the laminate with such adhesives it is necessary to provide a certain amount of time in which the solvent is allowed to evaporate from the adhesive before actual bonding occurs. This adhesive drying time tends to curtail the use of such solvent containing adhesives in applications which require the use of assembly line techniques in the fabrication of the laminates. It furthermore poses problems of solvent entrapment within the composite structure, which may lead to lower physical properties and odor problems.
Another drawback attendent upon the use of various types of adhesive relates to the use of adhesives which are of the curable type. These adhesives have a so-called "potlife" within which the adhesive must be used or it loses its utility. This potlife characteristic of the adhesive also tends to mitigate against the use of such adhesives for certain types of applications. The use of such adhesives also results in economic loss arising from the loss of adhesive which cannot be properly used during the potlife of such adhesives.
U.S. Pat. Nos. 2,502,304, 2,962,409 and 3,367,760 disclose processes for making laminates without the use of adhesives, but the laminates thus prepared have relatively little utility because of the relatively simple core geometries that can be produced using the process as disclosed in such patents.